/Engine/Plugins/Online/OnlineSubsystem/Source/Public/Interfaces/OnlineGroupsInterface.h
Any user but team owner can be denylisted including member, invitee, applicant and/or any other user unrelated to the team. Blocking follows "ignore" logic: blocked users shouldn't know for sure that they've been blocked. Blocked user will still be able to apply for membership and cancel pending application even while blocked.
void BlockUser ( const [FUniqueNetId](API\Runtime\CoreOnline\Online\FUniqueNetId) & ContextUserId, const [FUniqueNetId](API\Runtime\CoreOnline\Online\FUniqueNetId) & GroupId, const [FUniqueNetId](API\Runtime\CoreOnline\Online\FUniqueNetId) & UserId, const [FOnGroupsRequestCompleted](API\Plugins\OnlineSubsystem\Interfaces\FOnGroupsRequestCompleted) & OnCompleted )